A portable terminal such as a mobile telephone and a digital camera may include an illuminance sensor mounted in a liquid crystal panel to detect environmental brightness in order to control brightness (light emission amount) of a backlight device so that the brightness corresponds to the environmental brightness. This arrangement can reduce power consumption of the portable terminal and also improve visibility for the liquid crystal panel.
A typical illuminance sensor includes a silicon photodiode. Silicon photodiodes are widely used because they are compact and quick in response. A silicon photodiode, however, has a spectral-response characteristic which greatly differs from a spectral luminous efficacy for the human. Specifically, a silicon photodiode has a high sensitivity for light within an infrared region. A relation (photoelectric sensitivity) between an amount of received light and a photoelectric current depends on a wavelength of incident light, the wavelength having a relation (herein referred to as “spectral-response characteristic”) with photoelectric sensitivity.
There has thus been an increasing demand for a sensor, for use as an illuminance sensor, which includes a silicon photodiode and nonetheless has a spectral-response characteristic approximate to the spectral luminous efficacy for the human.
A known system for achieving a spectral-response characteristic approximate to the spectral luminous efficacy for the human is a system involving subtraction for respective currents flowing through a plurality of photodiodes having different spectral-response characteristics. Patent Literatures 1 and 2, for example, each propose an illuminance sensor based on a system similar to the above.
FIG. 14 is a circuit diagram illustrating a main arrangement of the respective illuminance sensors proposed in Patent Literatures 1 and 2.
As illustrated in FIG. 14, the illuminance sensors of Patent Literatures 1 and 2 each include a current mirror circuit.
The illuminance sensors each include photodiodes PD1 and PD2. The photodiode PD1 has a current Iin1 flowing therethrough in correspondence with environmental brightness. The photodiode PD2 has a current Iin2 flowing therethrough in correspondence with environmental brightness.
The illuminance sensors each include transistors QP1 and QP2, which constitute a current mirror circuit. The transistor QP2 has a collector current, which is a current (Iin1×α, where α is a random coefficient) corresponding to the current Iin1 through the photodiode PD1.
The photodiodes PD1 and PD2 differ from each other in spectral-response characteristic for wavelengths of light. The illuminance sensors each achieve a spectral-response characteristic approximate to the spectral luminous efficacy by subtracting, from the current Iin2 through the photodiode PD2, the current (Iin1×α) corresponding to an amount of the current Iin1 through the photodiode PD1.
Further, there has been known a method for converting a sensor output into a digital value with use of an analog-digital conversion circuit. Converting, for example, an output current into a digital value as such facilitates processing on software by a CPU or a microcomputer. An integrating analog-digital conversion circuit, in particular, characteristically achieves a highly precise resolution with use of a simple configuration. An integrating analog-digital conversion circuit is thus suitable for use in a device, such as an illuminance sensor, which requires both a low speed and a highly precise resolution (approximately 16 bits).
FIG. 15 is a circuit diagram illustrating a main arrangement of an illuminance sensor including an analog-digital conversion circuit.
As illustrated in FIG. 15, the illuminance sensor includes photodiodes PD1 and PD2. The photodiode PD1 has a current Iin1 flowing therethrough in correspondence with environmental brightness. The photodiode PD2 has a current Iin2 flowing therethrough in correspondence with environmental brightness.
The current Iin1 is subjected to an analog-digital conversion by an analog-digital conversion circuit ADC 1 into a digital value ADCOUT1, whereas the current Iin2 is subjected to an analog-digital conversion by an analog-digital conversion circuit ADC2 into a digital value ADCOUT2.
The illuminance sensor achieves a spectral-response characteristic approximate to the spectral luminous efficacy by outputting a value (ADCOUT2−ADCOUT1×α) which is obtained by (i) multiplying the digital value ADCOUT1 by a (where α is a random coefficient) and (ii) subtracting, from the digital value ADCOUT2, a value (ADCOUT1×α) corresponding to the digital value ADCOUT1.
Other than an illuminance sensor for detecting environmental brightness as described above, a portable terminal such as a mobile telephone and a digital camera may include a proximity sensor mounted in a liquid crystal panel for detecting whether a detection object (for example, a face) is present. Such a portable terminal, for example, (i) detects whether a face is close to the portable terminal, and thus (ii) turns a backlight device off in a case where a face is close to the portable terminal (during a telephone call) and turns the backlight device on in a case where a face is not close to the portable terminal (during a manual operation). This arrangement can reduce power consumption of the portable terminal.
The following describes a proximity sensor with reference to FIGS. 16, 17(a), and 17(b). FIG. 16 is a view schematically illustrating an arrangement of a typical proximity sensor. FIG. 17(a) is a waveform chart for proximity/non-proximity, detected by the proximity sensor, of a detection object, specifically for a case in which proximity of a detection object is detected. FIG. 17(b) is a waveform chart for proximity/non-proximity, detected by the proximity sensor, of a detection object, specifically for a case in which non-proximity of a detection object is detected.
As illustrated in FIG. 16, the proximity sensor includes a photodiode (PD), a light-emitting diode (LED), and a control circuit.
The light-emitting diode is driven by the control circuit and thus emits a particular light. The photodiode for receiving light has a current flowing therethrough in correspondence with an amount of received light, the current being detected by the control circuit. The proximity sensor produces proximity data (Data1−Data2), which corresponds to a difference between (i) data Data1 obtained while the light-emitting diode is driven and (ii) data Data2 obtained while the light-emitting diode is not driven.
In the case where a detection object (for example, a face) is present, the detection object reflects a large amount of light while the light-emitting diode is driven (see FIG. 17(a)). This increases the current through the photodiode, and thus causes the proximity data (Data1−Data2) to exceed a threshold Datath of the control circuit. The proximity sensor determines proximity of a detection object as a result.
In the case where a detection object is not present, on the other hand, only a little light is reflected by a detection object while the light-emitting diode is driven (see FIG. 17(b)). This decreases the current through the photodiode, and thus causes the proximity data (Data1−Data2) not to exceed the threshold Datath of the control circuit. The proximity sensor determines non-proximity of a detection object as a result.
It is a known technique to, for accurate detection of whether a detection object is present, cause a light-emitting diode to emit light within an infrared region, the light being included in hardly any amount in light from a fluorescent lamp or light under a dim, outdoor environment.
In such a proximity sensor, the proximity data (Data1−Data2), which corresponds to a difference between (i) data Data1 obtained while the light-emitting diode is driven and (ii) data Data2 obtained while the light-emitting diode is not driven, is inversely proportional to the square of a detection distance. The proximity sensor can thus be used as a distance measuring sensor which calculates a detection distance from the proximity data.